Environmental Factors Key to Autism

A new study published earlier this week will likely dump
buckets of fuel on the fire that rages in the debate over autism. For the past
decade, there has been ongoing vitriol spilled over the (potential)
relationship between early childhood vaccination and the development of autism.
The truth of the matter, however, is that there has been no definitive answer
to why some children become autistic and others do not. A recent study may help
guide researchers to resolve this open medical mystery.

According to this new study, conducted jointly by UCSF and
Stanford, and published Monday in the Archives of General Psychiatry,
environmental factors play a much larger role in causing autism than previously
thought—apparently, an even larger role than genetics.

The study, the largest of its kind to date, looked at 192
pairs of twins in California—all of which had at least one autistic sibling;
some of which had two. The results, after being wrung through complicated
mathematical modeling and analysis, are that genetics account for 38 percent of the risk of autism, and
environmental factors are to blame for the remaining 62 percent.

These findings are sure to provide firepower to those who
believe that vaccination can cause autism, people like Jenny McCarthy, the former
pinup model and self-proclaimed champion of children everywhere, who leads the
global anti-vaccine campaign via both personal
grandstanding and through her non-profit Generation Rescue. McCarthy and
others like her hold the belief that the MMR (Mumps, Measles, and Rubella)
vaccine leads to cancer. The MMR vaccine, introduced in 1971, is generally
considered standard preventative care for kids. But it is no stranger to
controversy.

McCarthy and others who believe autism is caused by the MMR
vaccine base their beliefs in a controversial and immensely influential 1998
study performed by Dr. Andrew Wakefield. Dr. Wakefield’s study has been largely
discredited, pronounced an “elaborate fraud” by the British Medical Journal (BMJ) and retracted
in 2010 by The Lancet, the same
publication that had originally published the study a decade earlier.

On the other side of the debate are those who have believed
autism is a highly inheritable disease—a group comprising the majority of the
medical community. Previous studies have suggested that autism is 90 percent
genetic, with only a slight environmental factor at play.

This study, however, blew these beliefs wide open.

Here’s why: Of the 192 twin sets, 54 were identical; 138
were fraternal. As a refresher, identical twins are nearly 100 percent
genetically identical; fraternal twins share the same genetic makeup only to
the extent any siblings do, so in the range of 50 percent.

If autism is, in fact, a primarily genetic disease, scientists
would expect that if one identical twin had the disorder, the other twin would
too, nearly 100 percent of the time. Among fraternal twins, if one twin was
autistic, the other should have a slightly higher-than-normal risk of being
autistic as well, but not huge.

However, the study found that autism occurred in both
siblings in 77 percent of male identical twins and in 50 percent of female
identical twins. Perhaps even more surprising were the rates among fraternal
twins: 31 percent of males and 36 percent of females.

When plugged into a mathematical equation taking into
account expected rates if autism were to be found entirely genetic- or
environmentally-caused, these results suggested that only 38 percent of the
autism cases could be attributed to genetic factors.

As for the 62 percent of cases attributable to environmental
factors, the authors of the study stressed that it remains very unclear what
those factors are. There is no proof that vaccines cause autism, but there are
also not that many other ideas floating around the medical community.

A
second study also published in the Archives of General Psychiatry found a
slightly elevated risk of autism in children whose mothers took antidepressants
during their pregnancy—but the increased risk was so slight as to be negligible.
Expecting mothers currently on antidepressants should not stop taking them,
said the study’s authors.

This and other studies are inconclusive, experts say.
However, the California twin study will change the way scientists think about
and research autism and its causes.

Dr. Clara Lajonchere, one of the authors of the twin study
told The New York Times that “much
more emphasis is going to be put on looking at prenatal and perinatal factors
with respect to autism susceptibility.” Dr Lajonchere also stated, “We need to
not just study the environmental factors, but the relation between the genes
and the environment.”

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